1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic powder for high density magnetic recording and, more particularly, to a method for manufacturing a magnetic powder for vertical magnetic recording.
2. Description of the Prior Art
Magnetic recording has been conventionally based on magnetization along the longitudinal direction of the plane of the recording medium. However, when an attempt is made to record at a high density in such a recording system, the demagnetizing field within the recording medium increases. Thus, it is difficult to achieve high density recording with this recording system.
To the contrary, in a vertical magnetic recording system, the demagnetizing field within the recording medium decreases even when the recording density is increased, so that this system may be considered to be suitable for high density recording. With such a vertical magnetic recording system, it is necessary that the axis of easy magnetization be normal to the surface of the recording medium. Among such recording media, there is known a recording medium which is obtained by mixing a magnetic powder with a binder and coating the mixture on a tape.
As such a magnetic powder, hexagonal ferrites such as barium ferrite (BaFe.sub.12 O.sub.19) are used. These hexagonal ferrite powders are in plate form and the axis of easy magnetization is normal to the plane of the surface so they are advantageous in that vertical orientation may be easily accomplished by magnetic field orientation processing or mechanical processing. However, for using the hexagonal ferrites for vertical magnetic recording powder, certain conditions must be satisfied.
For example, the hexagonal ferrites are too high in coercive force iHc (generally over 5,000 oersteds) to be recorded by the magnetic head. Thus, it is necessary to reduce the coercive force to a value suitable for vertical magnetic recording.
Further, it is preferred that the crystal size of the hexagonal ferrites be controlled within the range of 0.01-0.3 .mu.m for vertical magnetic recording. When the crystal size is less than 0.01 .mu.m, the ferromagnetism necessary for magnetic recording is not obtained, and when it exceeds 0.3 .mu.m, magnetic recording may not be achieved at high density.
It is further required that the hexagonal ferrites be homogeneously dispersed in a medium such as a paint.
A hexagonal ferrite fine powder which satisfies all of these requirements has not been available as yet. With the general ferrite powder manufacturing method in which the powder raw materials such as oxides, hydroxides, and carbonates are mixed and reacted in solid phase at a high temperature, ion substitution for controlling the coercive force may be possible and the magnetic characteristics of the obtained magnetic powder may be good. However, coagulation of the particles by sintering may not be prevented so that this method is not preferable for manufacturing the magnetic powder for magnetic recording.
Another representative method for manufacturing the hexagonal ferrites is the hydrothermal method. According to this method, an alkali of a high concentration is added to a solution containing ferrite-producing ions and they are reacted at a high temperature and pressure in an autoclave. With this method, individual ferrite particles are produced in a solution and separated from each other so that a powder with extremely good dispersibility is obtained. However, when the particle size becomes less than 0.3 .mu.m as desired, the degradation of the magnetic characteristics becomes significant. Furthermore, when ion substitution is performed for controlling the coercive force, products other than the desired product are produced.